In low pressure discharge lamps the discharge is formed between electrodes. These operate alternatively as anode or cathode, respectively, if the lamp is supplied with A.C., or permanently as anode or cathode in case of D.C. operation. Cathodes consist usually of a coiled tungsten wire which is coated with a mixture of alkaline earth oxides to enhance thermionic electron emission.
The life of a fluorescent lamp is mainly determined by the life of the cathode.
The physical behaviour of an oxide coated cathode is complex. Roughly the cathode exists in a sensitive equilibrium of thermionic emission and evaporation of emissive material.